The present invention relates to thermal ink jet printing apparatus and is concerned, more particularly, with the ink supply system of a thermal ink jet printhead.
A thermal ink jet printer has at least one printhead in which thermal energy pulses are used to produce vapour bubbles in ink-filled channels and so cause droplets of ink to be expelled from the channel orifices towards a recording medium. The thermal energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapour bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Some arrangement is usually provided to clean the channel orifices periodically while the printhead is in use and to close-off the orifices when the printhead is idle to prevent the ink in the printhead from drying out.
One form of thermal ink jet printer is described in EP-A-0 210 848. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information. As an alternative to providing each printhead with its own ink cartridge, the printheads can be supplied with ink from one or more supply tanks which need not be mounted on the carriage.
In another form of thermal ink jet printer, several printheads are accurately juxtapositioned to form a pagewidth array which remains stationary while the recording medium is moved at a constant speed in a direction perpendicular to the length of the array. Printers of that type are described in U.S. Pat. No. 4,463,359 to Ayata et al (see FIGS. 17 and 20). In those particular printers, the printheads are mounted on a common metal plate which functions as a heat sink, for efficient dissipation of heat generated when the printer is in operation.
U.S. Pat. No. 4,454,518 refers to the importance of temperature control in an ink jet printer and, in particular, the control of the ink temperature in a printer of the type that utilizes a piezoelectric transducer to cause the discharge of ink droplets from a print head. U.S. Pat. No. 4,929,963 describes the cooling of the printhead of a thermal ink jet printer by causing ink to flow through the printhead in a volume far greater than the volume required for printing purposes.
It has also been recognized that there is a need for periodic maintenance of the printhead of an ink jet printer by, for example, periodically cleaning the ink discharge orifices when the printer is in use and/or by capping them when the printer is out of use or is idle for extended periods. Maintenance stations for the printheads of various types of ink jet printer are described in, for example, U.S. Pat. Nos. 4,853,17; 4,739,340; 4,734,719; 4,600,931; 4,577,203 and 4,543,591 while an alternative arrangement for cleaning an ink jet printhead is described in U.S. Pat. No. 4,727,378.